crocks/First/resultToFirst.js

A collection of well known Algebraic Datatypes for your utter enjoyment.

/** @license ISC License (c) copyright 2017 original and current authors */
/** @author Ian Hofmann-Hicks (evil) */

var First = require('.')
var Result = require('../core/types').proxy('Result')

var curry = require('../core/curry')
var isFunction = require('../core/isFunction')
var isSameType = require('../core/isSameType')

var applyTransform = function (result) { return result.either(First.empty, First); }

// resultToFirst : Result b a -> First a
// resultToFirst : (a -> Result c b) -> a -> First b
function resultToFirst(result) {
  if(isFunction(result)) {
    return function(x) {
      var m = result(x)

      if(!isSameType(Result, m)) {
        throw new TypeError('resultToFirst: Result returning function required')
      }

      return applyTransform(m)
    }
  }

  if(isSameType(Result, result)) {
    return applyTransform(result)
  }

  throw new TypeError('resultToFirst: Result or Result returning function required')
}

module.exports = curry(resultToFirst)